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mr-scorpio

Modifying Jet engine IDLE thrust level in AIR file?

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Hi there,

I've seen this topic touched upon in the past, but have yet to see a precise EASY to understand answer.

 

As has been seen for years now with various jets, sometimes when the aircraft is sitting there idling, it will begin to creep & roll. If the aircraft is 'light', that roll can quickly get up to 30+kts if not holding onto the breaks.

 

I respectfully ask NOT to reply with unhelpful answers like .. ' some planes DO creep etc. blah blah.', or 'decrease thrust scaler' .. (Which is is just a frig & weakens the engine overall.)

 

ALL i would like to know please is WHAT to edit in the AIR file? It seems to concern table 1506 (which seems to be the one that can set the IDLE thrust LEVEL), so that I can REDUCE the idle thrust WITHOUT affecting the overall thrust levels or engine performance higher up the thrust levels.

 

It's a bit strange that MS didn't give an: Idle_Thrust= xxx in the cfg file when they did allow a maximum: 'static_thrust' setting.

 

Table 1506 in the air file seems to have three rows of data, but not knowing the correlation between these rows & the 19+ columns, I don't want to randomly poke around & potentially break things.

FSX has been out nearly 6 years now, are there some clear answers?

 

Many thanks in advance.

Al

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The attached figure shows the general relationship for the steady-state, commanded N1, N2, thrust and airflow fior FSX. There seems to be uncertainty about the purpose of Table 1505,

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Hi there MGH,

Many thanks for the interesting diagram, but it doesn't really answer my question.

I am tempted to try Mudpond's Airwrench, but why pay $20 to tweak one thing?, hence i'd rather just know what to tweak & what values 'by hand'

Cheers

Al

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Aircraft Airfile Manager is free. Google it

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Table 1506 is what you want.

Top row is the n1%,second at 0.0 mach.

Lower the 20~35 range valus as you see fit.

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Hi Johan,

Many thanks for this.

The numbers still don't look 'obvious' though.

To be precise, i'm trying to correct the superb Buccaneer from UKMil.

The Spey engines are set to maximum thrust of 11030lbs (easy to do in both the AIR & CFG files).

However when analysing the data in Mudpond's Airwrench (not paid for yet, so read only), is it showing the idle thrust as being 1168lbs ... Which would account for the creep.

After doing some research, the RR Spey at idle, produces only around 400lbs or so thrust.. THIS is what i want to change, and of course would stop the creep, even when the plane is 'light'.

 

So given the figures that i see in the entry 1506, what shall i change that will set 400lbs idle thrust, but still maintain a nice gradient up to 11030lbs thrust? (I don't want to break things so that the engines behave 'weirdly' & jump up in thrust.)

Many thanks

Al

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if you look to the table, you can see that the top row is the N1 percentage.

At idle, lets say the engines give 23% N1. So you are between 20-30 in the table somewhere.

With some trail and error you can with some educated guessing and trying just lowet the next row values for it. Let say its 4.00. I dont know yet of that is a % of the max thrust or some other strange value.

Lower it to 2.0 to reduce 50% thrust at between 20-30 equals to 25 % N1, and so on.

I often drop it really low for idle thrust, to prevent exactly that: creeping.

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Gross thrust is determined by looking up the value in Table 1506 for N1 and Mach Number and then multiplying it by the ratio between ambient total pressure and sea level pressure and then by the static_thrust from the .cfg file.

 

At sea level the ratio equals 1 so to get idle thrust at rest you need to set values in the second column (Mach = 0) corresponding to your idle N1 to 0.03888(= 400/10300).

 

Aircraft Airfile Manager displays tables graphically so you can immediately the effects and also “smooth-out” the curves.

Gross thrust is determined by looking up the value in Table 1506 for N1 and Mach Number and then multiplying it by the ratio between ambient total pressure and sea level pressure and then by the static_thrust from the .cfg file.

 

At sea level the ratio equals 1 so to get idle thrust at rest you need to set values for Mach = 0 corresponding to your idle N1 to 0.03888 (= 400/10300).

 

Aircraft Airfile Manager displays tables graphically so you can immediatelyaee the effects and also “smooth-out” the curves if neceswsary.

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Johan & Mgh.

Thank you for your Excellent Answers! - I shall try & tackle the issue now.

best

Al

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Gross thrust is determined by looking up the value in Table 1506 for N1 and Mach Number and then multiplying it by the ratio between ambient total pressure and sea level pressure and then by the static_thrust from the .cfg file.

 

At sea level the ratio equals 1 so to get idle thrust at rest you need to set values in the second column (Mach = 0) corresponding to your idle N1 to 0.03888(= 400/10300).

 

Aircraft Airfile Manager displays tables graphically so you can immediately the effects and also “smooth-out” the curves.

Gross thrust is determined by looking up the value in Table 1506 for N1 and Mach Number and then multiplying it by the ratio between ambient total pressure and sea level pressure and then by the static_thrust from the .cfg file.

 

At sea level the ratio equals 1 so to get idle thrust at rest you need to set values for Mach = 0 corresponding to your idle N1 to 0.03888 (= 400/10300).

 

Aircraft Airfile Manager displays tables graphically so you can immediatelyaee the effects and also “smooth-out” the curves if neceswsary.

 

Sorry to reopen such an old topic but this gives the best google hit to my question, originally regarding idle thrust but as always one question leads to another!

 

would someone (mgh) be able to explain the following anomaly? table 1506 mach 0 row

 

in my aircraft.cfg, static thrust is set to 115000lbs

 

when i follow the N1 through, I find that higher N1s using the calculation above means my static thrust level is exceeded. I want 3000lbs at idle 23%

 

3000lbs /115000lbs =0.026086

 

If I follow this equation through to work out what each mach value means in relation to thrust produced I end up with the following table

 

 

0.026086 *115000 = 3000lb

 

0.090000 *115000 = 10350lb

 

0.100000 = 11500lb

 

0.200000 = 23000lb

 

0.3 = 34500lb

 

0.400000 = 46000lb

 

0.600000 = 69000lb

 

0.800000 = 92000lb

 

1.000000 = 115000lb

 

1.083300 = 124579.5lb (N1 100 mach 0)

 

1.18970 = 136815.5 (N1 105)

 

1.218100 = 140,081.5 (N1 110)

 

 

So it seems higher N1s allow the thrust to be exceeded, shouldnt I only ever see a maximum value of 1 in the mach 0 row at the highest n1?

 

confused!

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